This disclosure relates to a method for decreasing nitrogen oxide (“NOx”) emissions from a fluidized bed combustion system. In particular, this disclosure relates to the selective injection of a reactant into the combustion system for reducing NOx levels in the gaseous products of combustion in the fluidized bed combustion system.
FIG. 1 is a schematic depiction of the prior art and shows a fluidized bed combustion system 100. In the circulating fluidized bed combustion system 100, fuel, bed material and possible sorbent material are fluidized in a furnace 2 with fluidizing air, which is introduced to the furnace 2 via an air introduction port 20. In general, air is introduced to the furnace 2 through various introduction ports located at different levels of the furnace 2, but for clarity, the FIG. 1 only depicts a single means for introducing air into the furnace 2. Exhaust gases produced in the furnace 2 and other particulate matter entrained in the exhaust gases are discharged through a solids separator inlet duct 4 in the elevated portions of the furnace 2 to a solids separator 8. In the solids separator 8, which is usually a cyclone, most of the particulate matter is separated from the exhaust gases and returned to the furnace 2 via a solids return duct 6.
The exhaust gases are led from the solids separator 8 through an outlet duct 14 to an exhaust gas duct 18, which comprises heat transfer surfaces (not shown) for cooling the exhaust gases and for producing steam that may be used for heating the fluidized air respectively. Exhaust gases produced in the furnace 2 generally contain NOx, which is environmentally unfriendly. It is therefore desirable to neutralize the NOx prior to venting any portion of the exhaust gases into the atmosphere. Urea, aqueous or anhydrous ammonia (hereinafter ammonia), or other reagents having an ammonium radical are generally used to neutralize NOx.
To reduce NOx emission levels, selective non-catalytic reduction (“SNCR”) methods and selective catalytic reduction methods (“SCR”) are employed. In SNCR methods, a reactant such as urea or ammonia is injected into the combustion system to react with the NOx, forming nitrogen (“N2”) and water (“H2O”). The reactant is generally injected through numerous ports at various locations across the combustion system including the furnace, the separator, and the duct connecting the furnace and separator.
With reference once again to the FIG. 1, the reactant (for the neutralization of NOx) is generally introduced into the fluidized bed combustion system 100 either in the inlet duct 4 via a port 22, or directly to the solids separator 8 via another port 24 or at the top 12 of the vortex finder 16 located in a dome at the upper end of the solids separator 8. Each of these points of introduction has drawbacks.
For example, inefficient utilization of the reactant often prevents the SNCR methods from obtaining the desired degree of decrease in NOx levels. For more efficient usage of the reactant, it is desirable to have a high residence time of the reactant in the system, a high degree of mixing of the reactant with the NOx-containing exhaust gases, and a low degree of mixing of the reactant with the particulate materials circulating in the system. Present systems often suffer from inefficient use of the reactant. For example, systems that inject the reactant into the furnace 2 and systems that inject the reactant into various locations across the inlet duct 4 may suffer from too much mixing of the reactant with the particulate materials and insufficient mixing of the reactant with the NOx-containing exhaust gases. Similarly, systems that inject the reactant into the solids separator 8 or at the top 12 of the vortex finder 16 may suffer from insufficient distribution and residence time and from insufficient mixing of the reactant with the NOx-containing exhaust gases. All such system have injection ports or lances that do not penetrate sufficiently into the bulk on the gas duct because of concerns with high temperature and clogging of the ports. Inefficient utilization of the reactant results in excessive use of the reactant, which adds to the cost of the SNCR method. Additionally, adding excessive amounts of the reactant can generate new pollution problems.
The high temperatures encountered in the furnace 2 and the solids separator 8 often limit the materials and the types (e.g., designs) of NOx reducing systems that can be introduced for reducing the NOx content in the exhaust gas stream. In addition, the high particulate content in the exhaust gas stream also results in a degradation of NOx reducing systems, thus reducing the life cycle of such devices and increasing the amount of maintenance that is to be conducted on the fluidized bed combustion system 100.
It is therefore desirable to have a system that permits sufficient distribution and mixing of the reactant with the NOx-containing exhaust gases to reduce the NOx content in the exhaust gas stream. It is also desirable to have a NOx reducing system that has a robust design that can withstand operating temperatures in the fluidized bed combustion system 100 and that can withstand the degrading effects of the particulates present in the exhaust gases.